This invention relates to a solid electrolyte type fuel battery.
A fuel battery converts chemical energy, obtained by the reaction between a fuel (a reducing agent) and air (an oxidizing agent), directly into electric energy, rather than taking it as heat. The fuel battery can serve as a highly efficient power generation system, because it carries out direct power generation and it is not restricted by the Carnot's cycle. The fuel battery comes in various types, and can be roughly divided into a phosphate type, a molten carbonate type, and a solid electrolyte type according to the solid electrolyte used. The temperatures used for these types are about 200° C., 600° C. and 1,000° C., respectively. The solid electrolyte type fuel battery, in particular, can utilize high waste heat, and thus shows a high thermal efficiency of about 60%.
The constituent materials for the solid electrolyte type fuel battery, as a single cell, are a solid electrolyte, a fuel electrode, and an air electrode. The materials generally used are yttrium stabilized zirconia (hereinafter referred to as YSZ), an NiO/YSZ system material, and a lanthanum manganese system material, respectively.
However, the open circuit voltage of a single cell is about 1 V, so that the series connection of the cells needs to be made by an interconnector for actual use. To support the respective constituent materials in terms of strength, a cylindrical battery generally requires the use of calcia stabilized zirconia (hereinafter referred to as CSZ) as a support pipe, while a flat plate type battery requires the use of the interconnector itself as a support plate.
The interconnector must fulfill the stringent requirements that it be too tight to allow passage of a gas; it be chemically stable in both oxidizing and reducing atmospheres at a high temperature of about 1,000° C.; it should not form an insulating layer upon reaction with other constituent material during battery production and in operation; it be highly electrically conductive and should undergo only electronic conduction without involving ion conduction; and its thermal expansion be comparable to that of other constituent material, such as YSZ.
As a material that satisfies the foregoing strict requirements, an LaCrO3 system material (hereinafter referred to as lanthanum chromite) is generally used. This material does not completely fulfill the required properties, and has much to be improved, particularly, in terms of burning properties and cracking due to expansion during reduction. The cracking during reduction occurs by the following mechanism: One surface of the interconnector is in contact with the oxidizing atmosphere, while the other surface of the interconnector contacts the reducing atmosphere. Thus, some oxygen is drawn out on the reducing side, causing expansion. As a result, a “warping” force works in the same material, resulting in fracture.
In the solid electrolyte type fuel battery, as noted above, severe requirements are imposed on the interconnector. Lanthanum chromite, in particular, is difficult to burn, and is usually burned only at a burning temperature higher than 1,600° C. in a reducing or vacuum atmosphere.
Thus, when a lanthanum chromite system interconnector is to be produced by a so-called integral burning method, the burning temperature has to be made high. This poses the problem that the porosity of the electrodes is lost to lower the output characteristics, and the problem that an insulating layer is formed at the interface between the lanthanum chromite and other constituent material, thereby deteriorating the performance. Therefore, there have been no cases in which batteries are produced by integral burning.
Film-forming methods for lanthanum chromite other than the burning method include the EVD (Electrochemical Vapor Deposition) method described in a report of an invention by Isenberg et al. (U.S. Pat. No. 4,490,444), and thermal spraying which is a common method. Both of these methods are problematical in the manufacturing cost, and they are not suitable for mass production.
Furthermore, lanthanum chromite has the property that it tends to expand in a reduced condition. Thus, when exposed to an oxidizing atmosphere and a reducing atmosphere, this material breaks owing to its own expansion difference.
Besides, the interconnector contacts all materials, and faces stress associated with the differences in thermal expansion coefficient among the respective constituent materials, as well as stress due to the aforementioned expansion during reduction. Unless the interconnector has high strength, it will break.